This application claims priority to: PCT Application No. PCT/GB99/02184 filed on Jul. 8, 1999; and Great Britain Application No. 9814775.4 filed on Jul. 9, 1998.
The present invention relates to an electron spin polarimeter.
The spin polarisation of an electron beam is a vector quantity which has two transverse components and one longitudinal component. Measurement of spin polarisation is commonly used in the study of electron scattering processes, in the study of interactions involving polarised atoms and in magnetic studies.
It is of interest to measure each of the three components of spin polarisation of an electron beam. However, known polarimeters are either sensitive to one, or both, of the transverse components, or to the longitudinal component, of the spin polarisation.
A known polarimeter utilises Mott Scattering to measure transverse components of beam polarisation. This polarimeter is commonly referred to as a Mott polarimeter. The Mott polarimeter takes advantage of a left-right scattering asymmetry which results from spin-orbit coupling when high-energy electrons are scattered at large angles from a gold foil. This type of polarimeter is capable of measuring both of the transverse components of spin polarisation of an electron beam, but is unable to measure the longitudinal component.
There are two known methods for measuring each of the three components of spin polarisation of an electron beam. In a first known method two polarimeters are used. The first of the polarimeters measures the transverse spin polarisation components of the electron beam in the normal way. The second polarimeter is displaced perpendicularly from the electron beam, and the beam is deflected through 90xc2x0 using electrostatic deflection apparatus known as a switchyard, so as to be incident upon the second polarimeter. The deflection of the beam through 90xc2x0 converts the longitudinal component of spin polarisation into a transverse component which is then measured by the second polarimeter. A disadvantage of this approach is that it requires the use of two polarimeters.
The second approach to measuring all three components of spin polarisation uses a single polarimeter and a spin rotator. In a first measurement the spin rotator is deactivated and the two transverse components of spin polarisation are determined. In a second measurement the spin rotator converts the longitudinal component into a transverse component and then this, together with the undisturbed transverse component, is measured. A disadvantage of this approach is that it requires the use of a spin rotator.
It is an object of the present invention to obviate or mitigate the above disadvantages.
According to a first aspect of the present invention there is provided a polarimeter for analysing the electron-spin polarisations of an electron beam, the polarimeter comprising first target means comprising a layer of material for scattering a beam of electrons in directions dependent upon the transverse spin-orientation of the incident electrons, first detector means for detecting the scattered electrons, second target means comprising a layer of ferromagnetic material, magnetising means for magnetising the ferromagnetic layer such that the second target means will transmit a beam of electrons at a rate dependent upon the longitudinal spin-orientation of the electrons, and second detector means for detecting the transmitted electrons.
The inventor has realised that a first polarimeter, which measures transverse components of spin polarisation of an electron beam may be combined in one apparatus with a second type of polarimeter which measures the longitudinal spin polarisation of a low energy electron beam. This second type of polarimeter comprises one or more thin magnetic layers sandwiched between non-magnetic layers, the transmission of a low energy electron beam by this combination of layers being dependent upon the longitudinal polarisation of the beam (see for example Lassailly Y., et al, Phys. Rev. B, 1994, Vol. 50, pg. 13054). For instance, the second type of polarimeter may comprise a single thin magnetic layer sandwiched between two non-magnetic layers, or more than two magnetic layers separated by non-magnetic spacers.
When measuring transverse spin polarisation, the electron beam may for example have an energy between 50 eV and 30 keV, and a suitable polarimeter would be, for example a Mott polarimeter such as a compact retarding field polarimeter, an absorbed current polarimeter, a LEED polarimeter, or alternatively a NIST type polarimeter (diffuse scattering polarimeter). In the case of a NIST polarimeter, repeated deposition of fresh gold onto the target is required. The energy of the electron beam during longitudinal polarisation measurement will generally be less than 200 eV. It is envisaged that in the preferred embodiment the energy of the electron beam detected by the first type of polarimeter will be relatively high compared to the second type.
The polarimeter is advantageous in that it allows measurement of each of the three components of spin polarisation of an electron beam using a single piece of apparatus, and without requiring a spin rotator.
Preferably, the layer of ferromagnetic material is sandwiched between front and back layers of an unreactive and non-magnetic material.
Preferably, the first target means comprises the front layer of the second target means. This configuration is advantageous because it allows the first target means to fulfil two roles, i.e. it can function as the first target means when transverse spin polarisation is to be measured, and it can also function as the front layer of the second target means when longitudinal spin polarisation is to be measured.
In the alternative, the first target means and the second target means may be separated from one another. For instance, the second target means might be located downstream (in relation to the electron beam direction) from the first target means. When a (low energy) measurement of longitudinal polarisation is to be performed either the first target means could be moved from the beam path or the electrons could be allowed to pass directly through the first target means so as to be incident at the second target means. Although the apparatus comprises two separate target means in this embodiment, the apparatus is still advantageous in relation to the prior art because it allows the construction of a single polarimeter which measures all components of spin polarisation.
The electron beam incident on the first target means will be both transmitted and scattered to some degree dependent upon the voltage applied to the target. At high voltage spin dependent scattering is more significant than at low voltage. Means are preferably provided for switching a voltage applied to the first target means between such relatively high and relatively low voltages. Preferably, means are provided for switching a voltage applied to the first target means between a high voltage at which electrons are predominantly scattered by the target and a low voltage at which electrons are predominantly transmitted by the first target means towards the ferromagnetic layer.
As mentioned above suitable energy ranges may be between 50 V and 30 kV for the transverse polarisation measurement, and less than 200 V for the longitudinal polarisation measurement. Preferably the energy for the transverse measurement is selected to be suitable for a Mott polarimeter and may typically be 10-20 kV. A typical energy level for the longitudinal polarisation measurement would be below 10 V.
Preferably, the scattered electrons are decelerated by a retarding electrical field before reaching the first detector means. The retarding electric field may be established between electrodes having for instance hemispherical, conical or cylindrical shapes.
Various materials may be used for the first and second target means. Examples of suitable materials for the first target means are gold, thorium and uranium, of which gold is particularly suited because it is unreactive and readily deposited by conventional evaporation techniques.
Any ferromagnetic pure metal or alloy that exhibits remanent magnetisation out of plane may be suitable for use as the ferromagnetic layer. One particular material which is suitable for use as the ferromagnetic layer is cobalt (since it is readily deposited on gold), but any other material exhibiting the appropriate anisotropy may be used (such as iron and nickel for instance). For instance, in one preferred embodiment the target means comprises a layer of cobalt sandwiched between layers of gold. Other hypothetically suitable target structures include a layer of iron sandwiched between layers of thorium or uranium (this has not yet been tested). The magnetic layer must be sufficiently thin that it does not magnetise in its plane. A suitable thickness would for instance be 10 xc3x85 of cobalt.
Preferably, the second detector means comprises a retarding field energy analyser, for instance incorporating a channel electron multiplier or micro-channel plate detector.
The magnetising means is preferably an electromagnet.
According to a second aspect of the present invention there is provided a method of determining the longitudinal and transverse components of spin-polarisation of an electron beam, the method comprising directing the beam of electrons accelerated to relatively high energy at a first target means comprising a layer of material for scattering a beam of electrons in directions dependent upon the transverse spin polarisations of the incident electrons, detecting electrons which are scattered from the first target means, and directing the beam of electrons decelerated to relatively low energy at a second target means comprising a layer of ferromagnetic material magnetised such that the second target means will transmit a beam of electrons at a rate dependent upon the longitudinal spin-orientation of the electrons, and detecting electrons transmitted by the second target means.